In most fuel supply systems applicable to internal combustion engines, fuel injectors are used to direct fuel pulses into the engine combustion chamber. A commonly used injector is a closed-nozzle injector which includes a nozzle assembly having a spring-biased nozzle valve element positioned adjacent the nozzle orifice for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The nozzle valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. The nozzle valve is positioned in a nozzle cavity and biased by a nozzle spring to block fuel flow through the nozzle orifices. In many fuel systems, when the pressure of the fuel within the nozzle cavity exceeds the biasing force of the nozzle spring, the nozzle valve element moves outwardly to allow fuel to pass through the nozzle orifices, thus marking the beginning of injection.
In another type of system, such as disclosed in U.S. Pat. No. 5,819,704, the beginning of injection is controlled by a servo-controlled needle valve element. The assembly includes a control volume positioned adjacent an outer end of the needle valve element, a drain circuit for draining fuel from the control volume to a low pressure drain, and an injection control valve positioned along the drain circuit for controlling the flow of fuel through the drain circuit so as to cause the movement of the needle valve element between open and closed positions. Opening of the injection control valve causes a reduction in the fuel pressure in the control volume resulting in a pressure differential which forces the needle valve open, and closing of the injection control valve causes an increase in the control volume pressure and closing of the needle valve.
Internal combustion engine designers have increasingly come to realize that substantially improved fuel supply systems are required in order to meet the ever increasing governmental and regulatory requirements of emissions abatement and increased fuel economy. Specifically, it is well known that improved control of fuel metering, i.e. the rate of fuel flow into the combustion chamber, is essential in reducing the level of emissions generated by the diesel fuel combustion process while minimizing fuel consumption. As a result, many proposals have been made to provide metering, or injection rate, control devices in closed nozzle fuel injector systems.
Piezoelectric devices are desirable for use as valve actuators for several reasons. One being that the devices allow for precise metering and control of small quantities of pressurized fuel. Another desirable feature is that piezoelectric actuators have reliable characteristics when calibrated properly and precisely. However, in a fuel injection valve, the amount of displacement of a piezoelectric element necessary to move the valve element through its valve stroke is very small. Therefore, any slight unintended separation between the piezoelectric elements or layers forming the piezo stack may interfere with effective stack expansion and/or the initial force on the valve thereby possibly adversely affecting fuel injection timing and metering, regardless of the accuracy of the initial calibration. Although piezo stacks are initially preloaded using some mechanism, such as pulling devices, e.g. nut and washer assemblies including a center rod, outer rods and/or outer cages, that pull the ends of the stack toward one another in compression, these preloading device do not provide sufficient preload on the stack throughout operation of the injector.
In addition, establishing an accurate interface between a piezo actuator and movable valve element can be difficult and costly due to small strokes and large forces associated with piezoelectric actuators. Stack-up tolerances due to the assembly of various components also make it difficult to create a match or flush interface between the actuator and valve element. At least one injector manufacturer has produced a piezoelectric injector which uses a hydraulic chamber, between the piezo actuator and the servo injection control valve, filled with low pressure drain fuel to equalize minimal manufacturing tolerances while also compensating for temperature-induced and wear-induced changes in length.
Also, the required size (cross section of the stack) of the piezoelectric elements forming the piezo stack is proportional to the valve opening force. With larger injectors, where the injector needle diameter is larger, a larger size control valve is necessary to reach the desired control chamber pressure dynamic. High opening forces are required to open these larger control valves at high pressures, thereby requiring larger stacks. However, larger piezo stacks are more expensive and less widely available.
U.S. Pat. No. 6,837,221 to Crofts et al. discloses a servo-controlled fuel injector nozzle assembly having feedback control. The injector includes a piezoelectric actuator to actuate a valve member controlling fuel flow from a control volume positioned adjacent one end of a needle valve element to thereby control movement of the needle valve element. This design may not adequately provide preload on the actuator stack throughout operation and does not compensate for thermal expansion, wear and manufacturing tolerances.
Therefore, there is still a need for a simple, improved piezoelectric fuel injector which is capable of maintaining sufficient piezo stack preload throughout operation to ensure effective control over fuel injection.